The present invention relates to a method for ensuring capture of solid particles of great fineness, as well as to a device for carrying out this method.
Numerous industries use solids in pulverulent form. This is particularly the case of industries manufacturing paints, cosmetic and dermatological products, and pharmaceutical products. For example, the pharmaceutical industry, but also the cosmetics industry, requires novel galenic forms in order to improve the service rendered by the molecules of therapeutic or dermatological interest. In particular, it is seeking the means for effecting a rapid dissolution of these molecules, which are in the form of solid powder under usual conditions, within biological fluids such as blood or lymph. To that end, it is necessary either to modify the morphology of the solid, or to reduce the granulometry of the powder very considerably, or to combine these two actions. Numerous works are also carried out with a view to elaborating complex medicaments allowing a slow and regular absorption of the active molecule (delayed-action drug).
It is known, by numerous Patents and scientific publications, that microparticles can be obtained, with a granulometry generally included between 1 mm and 10 mm, and nanoparticles with a granulometry generally included between 0.1 mm and 1 mm, by using different methods of crushing or precipitation including, in particular, those employing supercritical fluids, the particles thus generated in that case being dispersed either in a liquid phase or in a gaseous phase, compressed or not, or in a supercritical fluid. Capturing of these particles is an operation which is always difficult, whatever the medium within which they are dispersed.
Supercritical fluids, and particularly supercritical carbon dioxide, are widely used to produce very fine powders capable of dissolving very rapidly by ingestion through the respiratory tracts. Supercritical fluids are also used for obtaining complex particles constituted by mixtures of different morphologies of the active principle and of an excipient, such as microspheres or microcapsules.
It will firstly be recalled what such a supercritical fluid is.
In effect, it is known that bodies are generally known in three states, namely solid, liquid or gaseous and one passes from one to the other by varying the temperature and/or the pressure. Now, there exists a point beyond which one can pass from the liquid state to the gas or vapour state without passing through a boiling or, inversely, through a condensation, but continuously: this point is called the critical point.
It is also known that a fluid in supercritical state, i.e. a fluid which is in a state characterized either by a pressure and a temperature respectively higher than the critical pressure and temperature in the case of a pure body, or by a representative point (pressure, temperature) located beyond the envelope of the critical points represented on a diagram (pressure, temperature) in the case of a mixture, presents, for very numerous substances, a high solvent power with no comparison with that observed in this same fluid in the state of compressed gas. The same applies to so-called xe2x80x9csubcriticalxe2x80x9d liquids, i.e. liquids which are in a state characterized either by a pressure higher than the critical pressure and by a temperature lower than the critical temperature in the case of a pure body, or by a pressure greater than the critical pressures and a temperature lower than the critical temperatures of the components in the case of a mixture (cf. the article by Michel PERRUTxe2x80x94Les Techniques de I""Ingxc3xa9nieur (Engineering Techniques) xe2x80x9cExtraction by supercritical fluid, J 2 770-1 to 12, 1999xe2x80x9d).
The considerable and modulatable variations of the solvent power of the supercritical fluids are, furthermore, used in numerous methods of extraction (solid/fluid), of fractionation (liquid/fluid), of analytical or preparative chromatography, of treatment of materials (ceramics, polymers) and of particle generation. Chemical or biochemical reactions are also made in such solvents. It should be noted that the physico-chemical properties of carbon dioxide as well as its critical parameters (critical pressure: 7.4 MPa and critical temperature: 31xc2x0 C.) make it the preferred solvent in numerous applications, all the more so as it does not present any toxicity and is available in very large quantities at very low price. Non-polar solvent, carbon dioxide taken to supercritical pressure sometimes has a co-solvent added thereto, constituted in particular by a polar organic solvent whose function is considerably to modify the solvent power, especially with respect to molecules presenting a certain polarity, ethanol often being used to that end. However, certain compounds are more favourably extracted by a light hydrocarbon having from 2 to 5 carbon atoms, and more favourably, from 2 to 4 carbon atoms, at supercritical pressure.
Among the methods allowing very fine particles to be obtained by means of a fluid at supercritical pressure, the method known under the designation of xe2x80x9cRESSxe2x80x9d will be particularly retained, according to which a solution of the product to be atomized is expanded very rapidly in a supercritical fluid, and the anti-solvent method of the type of the so-called xe2x80x9cSASxe2x80x9d, xe2x80x9cSEDSxe2x80x9d, xe2x80x9cPCAxe2x80x9d, xe2x80x9cASESxe2x80x9d methods, consisting in pulverizing a solution of the product in an organic or aqueous solvent within a stream of fluid in supercritical state.
These methods allow a powder to be obtained, formed by very fine particles which are dispersed within a gaseous stream at low pressure (RESS method) or at high pressure (SAS method). Other methods known in the prior state of the art also make it possible to generate very fine particles within a liquid, by precipitation, by recrystallisation or by mechanical crushing action.
The collection of these particles is then a very delicate operation, especially when it is desired that productions be large-scale.
Various methods allowing fine particles to be collected within a liquid or gaseous stream, are, of course, known. The most currently used ones are those employing filters constituted by woven or non-woven filtering materials which make it possible to capture the finest particles including those whose diameter is included between 0.1 xcexcm and 1 mm.
The filters also present a notorious drawback, insofar as the recovery of the particles that they have fixed as well as their possible subsequent re-use, are operations which are particularly difficult to carry out as long as it is desired to respect the rules imposed in the pharmaceutical industry.
The present invention has for its object to propose a method, as well as means for carrying out this method, making it possible easily to capture such particles and which, in addition, lend themselves to continuous operation on an industrial scale.
The present invention thus has for its object a method for capturing very fine particles present in a fluid flux in the liquid, gaseous or supercritical state, characterized in that it comprises the steps consisting in:
causing said flux to pass through a filtering element;
stopping the emission of said flux;
countercurrent sweeping the filtering material with a carbon dioxide flow under pressure, so as to entrain the particles deposited on the filtering material;
countercurrent expanding the flux, so as to trap the particles within a solid carbon dioxide snow-type mixture formed during its expansion.
The carbon dioxide flux used during sweeping will in particular be at a supercritical pressure. Furthermore, it may be advantageous to cool the carbon dioxide flow circulating in countercurrent, before expansion, so as to increase the quantity of solid of carbon dioxide snow type generated during the subsequent expansion.
In a form of embodiment of the invention, in which the particles will have been generated with the aid of a method employing an organic solvent, particularly of anti-solvent type, there will be percolated, in the normal direction of the flux, the particles collected by the filtering element with a fluid at supercritical pressure, before effecting the countercurrent sweeping, in order to eliminate the solvent present on and in the particles.
By a simple evaporation of the solid mixture, the invention makes it possible easily to obtain a dry and non-agglomerated powder.
The invention is also advantageous from an industrial standpoint, as it makes it possible successively and periodically to send the flux of fluid within which the particles are dispersed, towards a plurality of capture chambers, the particle production operation itself being conducted continuously.
The present invention also has for its object a device for capturing fine particles contained within a flux of liquid, gaseous or supercritical fluid, characterized in that it comprises:
at least one capture chamber comprising means for admission of said flux,
means for receiving the particles contained in the flux,
means for injecting in the capture chamber, through the receiving means, a flow of carbon dioxide under pressure in countercurrent with respect to the preceding flux,
means for placing the capture chamber in communication with a receptor chamber with the interposition of means for effecting an expansion of the flux in countercurrent, in the receptor chamber.
The capture device may comprises a plurality of capture chambers and commutation means making it possible to connect in turn each of these chambers to particle production means.
The means for receiving the particles may be constituted by filtering elements comprising, for example, going in the normal direction of the flux, a disc made of perforated metal, a filter made of microfibers and a disc of sintered material of greater porosity.